In the operation of cranes having electric motor hoist drives and hoist traversing drives, the load on the hoist lifting system and the hoist traversing system routinely varies from a very low value to a large value at rated full load (or greater) of the hoist lifting or traversing system. In view of these extreme variations in load conditions, it is often desirable to operate the motor drives under light or no-load conditions at higher speeds than are possible at high load conditions. However, in wound rotor alternating current drive motors connected to a 60 hertz alternating current line source, which are commonly used in hoist applications, motor speeds in excess of the rated motor no-load speed at 60 hertz are not possible. Direct current motor drives are more flexible in this respect, in that D.C. motors using series wound fields have the ability under light loads to increase speed in excess of that possible under heavily loaded conditions. This is due to the field weakening effect due to low current draw of the motor armature under light loads. This inherent operating characteristic of a D.C. motor allows the high speed retrieval of an empty hoist hook of the crane to thereby minimize non-load carrying time and increase the productivity of the crane. However, precise speed control of D.C. motors is difficult due to inertia of their heavy rotors. Further, D.C. motors have a high initial cost and are costly to maintain.
Alternating current adjustable frequency drives for various types of motor applications have also been in use for some time. More recently, adjustable frequency motor drives have been developed for hoist applications, however, due to exacting load control requirements required by hoists, various problems with adjustable frequency drives have prevented their wide spread use. One problem with adjustable frequency drives has been their lack of reliability in producing the necessary torque to control the load at the initiation of hoist operation. A recently developed solution to this problem involves the maintaining of the hoist brake engaged at the beginning of motor operation, applying power from the adjustable frequency drive at a low frequency only sufficient to produce the current and thereby the torque necessary to control the load on the hoist when the brake is released, and releasing the brake only after the necessary current level is detected. Another problem is that if the speed of the motor deviates from its rated speed relative to the frequency of the applied power, i.e., if the slip speed at which the motor produces maximum torque is exceeded, the motor torque is greatly reduced. Thus, should the motor speed not follow a change in the frequency of the power supply, the motor torque would fall below that required to control the load. A further drawback of adjustable frequency drives is that they do not utilize their adjustable frequency ability to increase the motor speed under light load conditions. The invention disclosed herein is directed to this latter deficiency in adjustable frequency drives.